Preheat the oven to 400°F (200°C). Line a baking sheet with foil and set it aside.

Slice the spaghetti squash in half lengthwise, then use a spoon to scoop out the seeds.

Rub the cut sides of the squash with 1 tablespoon of the oil, then sprinkle with salt and pepper.

Place the squash cut side down on the baking sheet and roast for 40-50 minutes, until a fork slides in easily but the squash still holds its shape. Don't over-roast, or the strands will turn mushy in the soup. Let the squash cool until you can handle it comfortably.

Scrape the flesh out of the shells with a fork, using long strokes to create spaghetti-like strands (see the step-by-step photos in the post). Set the strands aside.

While the squash cools, heat the remaining 1 tablespoon of oil in a large Dutch oven or pot over medium heat. Add the onion, carrots and celery and cook for 4-5 minutes, until starting to soften. Add the garlic, Italian seasoning, oregano, ½ teaspoon salt and ¼ teaspoon pepper and cook for 1-2 minutes more, stirring constantly.

Add the chicken broth and shredded chicken and bring to a boil. Reduce the heat and simmer for 15 minutes.

Stir in the spaghetti squash strands and simmer for 5 minutes, just until heated through. Taste, adjust the salt, and serve hot.